In a previously proposed digital VTR, a digital video signal is recorded onto a magnetic tape after it is first compressed in accordance with a DCT (Discrete Cosine Transform) technique and recorded in accordance with a variable length encoding technique. Such a digital VTR is capable of recording video signals in two different modes. In a first mode, the digital VTR records the well-known NTSC television broadcast signal, or the like. This mode is referred to herein as an SD mode, in which the video signal is recorded at a rate of 25 Mbps. In a second recording mode, the digital VTR records an HDTV signal. This second mode is referred to herein as the HD mode, in which the video signal is recorded at a rate of 50 Mbps. Techniques for recording what is referred to as a transport packet, which is formatted as an MPEG2 signal, are currently being proposed.
The MPEG2 format allows a plurality of different programs to be transmitted as a time division multiplexed encoded data stream. The fundamental data structure for organizing and conveying these multiplexed programs to their respective destinations is referred to as a transport packet.
Each transport packet has a fixed length of 188 bytes, and it comprises a header portion and a payload portion. The data of the header portion identifies the content of the transport packet. The digital VTR uses this header portion to select a designated program from the multiplexed program data stream.
FIGS. 22A-22D illustrate the contents of the transport packet. As shown in FIG. 22A, every transport packet includes a header portion followed by a payload portion. The payload portion corresponds to the contents of the video program. As shown in FIG. 22B, the header comprises: a sync code of eight bytes; a transport error indicator which indicates the presence or absence of errors in a packet; a payload unit start indicator which indicates the start of a payload unit; a transport priority code which indicates the importance of a corresponding packet; a packet identification (PID) code which indicates a particular attribute of the packet; a transport scramble control code which indicates whether the data of the payload portion has been scrambled; an adaptation field control code which indicates the presence or absence of an adaptation field; a cyclic counter that determines whether a part of the packet has been abandoned midway during transmission; and an adaptation field to which either additional identifying information or dummy bytes can be inserted.
As shown in FIG. 22C, the adaptation field is composed of several codes. The first code is an adaptation field length code that indicates the data length of the adaptation field. The next code is a discontinuous indicator code that changes its contents to indicate that a system clock has been reset. Following the discontinuous indicator code is a random access indicator code that indicates an entry point for random access. The random access indicator code is followed by a priority stream elementary indicator code that designates a portion or the entirety of the payload portion as important. The final portion of the adaptation field is designated as an optional field.
As shown in FIG. 22D, the optional field is composed of several codes. These codes are a PCR, an OPCR, a splice count down, a transport private data length and transport private data, an adaptational field extension length, and an optional field. The PCR code includes a time stamp for setting and calibrating a time value. A Phase Locked Loop (PLL) uses the PCR code to generate a system clock of 27 MHz, for instance. In order to accurately decode and reproduce the program data, the corresponding time base stored in the PCR field must be maintained with as little deviation as possible.
FIGS. 23A and 23B illustrate the manner in which an MPEG2 transport packet is recorded by the previously proposed digital VTR. A desired program (for example, program A) is selected from a time-division multiplexed data stream of programs A, B, and C. Assuming that a data rate of programs A, B, and C is equal to, for example, 30 Mbps and a substantial rate of the selected program is equal to 10 Mbps, a rate conversion from 30 Mpbs to 10 Mbps is executed in a rate converting buffer.
FIG. 24 illustrates such a rate converting buffer 102. The transport packet of the selected program is supplied to an input terminal 101 of rate converting buffer 102, which reduces the input program data rate to 1/3 of its original value. Thus, the rate is reduced from 30 Mbps to 10 Mbps. The rate converted transport packet is supplied from an output terminal 103 to a digital VTR.
Since a recording rate in the SD mode of the digital VTR is equal to 25 Mbps, by performing the rate conversion as mentioned above, the transport packet can be recorded as it is by the digital VTR.
By performing the above-discussed rate conversion to the selected program, the previously proposed digital VTR system also changes the time base of the selected program. Nevertheless, since the PCR code of the selected program still reflects the time base of the program before it was rate converted, a subsequent decoding operation that relies on this inaccurate PCR code will result in a poor reproduction of the rate-converted video signal.
The MPEG2 format provides for three frames of data: an I frame which was intra-frame encoded, a P frame which was forward direction prediction encoded, and a B frame which was bi-directionally prediction encoded. In a variable speed reproducing mode, since the reproducing head does not traverse the entire length of each track, the data of the continuous frames cannot be obtained. Thus, the data of the P and B frames cannot be decoded. Only the data of the I frame which was intra-frame encoded can be decoded. Therefore, only the data of the I frame is used in the variable speed reproducing mode of the previously proposed digital VTR system.
However, when the transport packet is supplied to the digital VTR for recording, the packets that include the I frame cannot be obtained entirely in the variable speed reproducing mode. A positional relation in which the data of the I frame has been recorded is uncertain. Therefore, the data of the I frame corresponding to a specific portion of a picture plane is dropped out at the time of the variable speed reproduction and a picture quality in the variable speed reproducing mode deteriorates.